hm, I found it, i think.
my card has a problem with the FOGGING_PER_PIXEL option.
I used that, because my ground is a 600x600 pixel big plane containing 2 triangles and the result was the best.
I set this to FOGGING_PER_VERTEX and see the ground again.
the same moment something new, strange occured: I have, as you may see on the sreens, the fog at the top border of the screen. when I walk north, its getting darker and darker until I come near the border of the plane. there the fog is gone and it gets lightened again.
dunno if it has something to do with the PER_VERTEX option and my simple plane.